Dielectric spectroscopy experiments are carried out in the ferroelectric phase of rubidium dihydrogen phosphate $\left({\mathrm{RbH}}_2{\mathrm{PO}}_4\right),$ within the frequency range $0.1\hspace{0.5em}\mathrm{Hz}<f<\mathrm{}1\mathrm{}\hspace{0.5em}\mathrm{MHz}.$ A strong dielectric time decay (ageing) dominates after thermal equilibration. However, spectra taken after long dwelling times reveal a significant dispersion below the fundamental piezoelectric resonance frequency $f_{\mathrm{res}}.$ The permittivity of the well-aged sample is found to decrease linearly with the logarithm of the frequency. Not detectable above the paraelectric-ferroelectric phase transition temperature $T_c,$ the dispersion is observed both in the range $T_c>T>\mathrm{}{T}_f\approx 117\hspace{0.5em}\mathrm{K}$ of the anomalously high domain wall contribution and in the low-temperature range $T<\mathrm{}{T}_f$ corresponding to the frozen-in domain-wall response. The result indicates that the concept of weak pinning in the context of the theory of elastic interfaces in disordered media may be applicable to describe the interaction between the ferroelectric/ferroelastic domain walls in ${\mathrm{RbH}}_2{\mathrm{PO}}_4$ and randomly distributed impurities.

• Received 25 October 2001

DOI:https://doi.org/10.1103/PhysRevB.65.134102